The present invention resides generally in the field of separations of acids from other compounds, and polymers useful for such separations. In particular, the invention relates to the separation of acids from other compounds using ampholytic base polymers.
It is often necessary to separate acidic compounds such as carboxylic acids from other compounds by treating mediums containing their admixture. As examples, citric and lactic acid are manufactured by fermentation in large scale worldwide. Such fermentations provide broths containing sugars and other compounds from which the desired acids must be separated for downstream use. Where such high volume manufacture is undertaken, it is extremely important to keep product recovery costs to a minimum.
Recent recovery work has focused on the use of solid polymeric materials to separate carboxylic acids from other components of fermentation broths. In this approach, the fermentation broth is treated with the solid polymer in a liquid-solid interaction which results in the separation of the acid from the other broth components. To date, a variety of solid polymers for such liquid-solid separation processes have been proposed.
For example, Kawabata et al., in U.S. Pat. No. 4,323,702, describe a process for recovering carboxylic acids with a material of which the main component is a polymeric adsorbent having a pyridine skeletal structure and a cross-linked structure. The carboxylic acid is adsorbed on the adsorbent, and then desorbed using a polar organic material such as an aliphatic alcohol, ketone or ester.
Kulprathipanja et al., in U.S. Pat. Nos. 4,720,579, 4,851,573, and 4,851,574, teach the use of solid adsorbents including a neutral, noniogenic, macroreticular, water-insoluble cross-linked styrene-poly(vinyl)benzene, a cross-linked acrylic or styrene resin matrix having attached tertiary amine functional groups or pyridine functional groups, or a cross-linked acrylic or styrene resin matrix having attached aliphatic quaternary amine functional groups.
South African Patent Application No. 855155, filed Jul. 9, 1985, describes other processes in which acids were recovered from their aqueous solutions. In an adsorption step, the acid-containing solution was passed through a column containing an adsorber resin consisting of a vinylimidazole/methylene-bis-acrylamide polymer, a vinylpyridine/trimethylolpropane tri-methacrylate/vinyltrimethylsilane polymer, a vinylimidazole/N-vinyl-N-methylacetamide/methylene-bis-acrylamide polymer, Amberlite IRA 35 (Rohm and Haas-acrylate/divinylbenzene based polymer containing dimethylamino groups), or Amberlite IRA 93 SP (Rohm and Haas) or Dowex MWA-1 or WGR-2 (Dow Chemical) (these latter three being styrene/divinylbenzene based polymers containing dimethylamino groups). To desorb the acid, water, usually at a temperature of 90xc2x0 C., was allowed to pass through the column.
International Applications PCT/US92/02107 filed Mar. 12, 1992 (published Oct. 1, 1992, WO 92/16534) and PCT/US92/01986 filed Mar. 12, 1992 (published Oct. 1, 1992, WO 92/16490) both by Reilly Industries, Inc., disclose recovering lactic and citric acid, respectively, using a divinylbenzene crosslinked vinylpyridine or other similar resin.
U.S. Pat. No. 5,412,126 describes processes in which citric acid is adsorbed on a base resin and then stripped using an alkylamine. The free acid is recovered by dewatering the material and driving the amine off with heat. U.S. Pat. No. 5,032,686 describes a process in which citric acid is separated from sugars using an acid resin, whereas U.S. Pat. No. 5,382,681 describes a process in which a citric acid solution containing another compound is first treated with base to convert the citric acid to trisodium citrate, whereafter the basic medium is passed over a base resin to separate other compounds.
Czechoslovakian Inventor""s Certificate No. 255497 B1, published on Jul. 16, 1997 and granted on Dec. 15, 1988, describes the use of a swellable resin containing carboxymethylpyridine functional groups to sorb anions and cations.
Despite the above-described work, there remain needs for improved, effective processes for treating mediums to separating acids from other compounds. The present invention addresses these needs.
A feature of the invention is the discovery that ampholytic base polymers can be used with advantage to treat mediums to chromatographically separate acids from other compounds. Accordingly, a preferred embodiment of the invention provides a process for treating a medium to separate at least one acid from at least one other compound, which includes contacting the medium with an ampholytic base polymer under conditions effective to separate the acid(s) from the other compounds. Preferred processes of the invention are chromatographic in nature, and involve separations achieved using a solid stationary phase containing an ampholytic base polymer. The ampholytic base polymer used in the present invention can include individual repeating units carrying both a positive and negative charge (so-called ampholytic inner salts), or alternatively or in addition can include some repeating units carrying only a negative charge and some repeating units carrying only a positive charge (so-called ampholytic ion pairs). Desirable ampholytic base polymers for purposes of the present invention include, for example, those encompassed by the formula
Pxe2x80x94X+xe2x80x94(Y)nxe2x80x94zxe2x88x92
wherein P represents a polymer backbone, X+ is a positively-charged group, Y is a linking group covalently linking X+ and Zxe2x88x92, n=0 or 1, and Zxe2x88x92 is a negatively-charged group. The ampholytic functions xe2x80x94X+xe2x80x94(Y)nxe2x80x94Zxe2x88x92 are preferably provided by pendant functions containing positively-charged tetravalent nitrogens (i.e. N+) wherein the nitrogens are covalently bonded to negatively-charged groups xe2x80x94(Y)nxe2x80x94Zxe2x88x92. Thus, preferred stationary phases for use in the invention have pendant functions containing the group

The tetravalent nitrogen, N+, can occur in a cyclic amino group or an acyclic amino group. In certain preferred polymers for use in the invention, this nitrogen occurs in a heterocyclic ring, such as that provided by modified pyridyl groups of polyvinylpyridines, especially 2- or 4-vinylpyridines.
Particularly preferred inventive processes employ a continuous contacting apparatus including a plurality of contacting zones (e.g. columns) filled with the ampholytic base polymer. A separation zone is established including a plurality of the contacting zones together containing a sufficient amount of the polymer to achieve substantial separation of the acid(s) from the other compound(s) in the medium. An elution zone is also established after achieving such separation, from which a product eluent is obtained containing either the acid(s) or the other compound(s).
For example, in a process for recovering an acid from a fermentation broth, the product eluent will contain the fermentively-produced acid separated from other undesirable components of the fermentation broth, for example sugars. On the other hand, in a process for removing an acid impurity from a sugar product, the product eluent will contain the sugar separated from the acid impurity.
A still more preferred manner of carrying out processes of the invention includes the steps of:
(a) providing a plurality of contacting zones containing an ampholytic base polymer;
(b) sequentially processing the contacting zones through a chromatographic separation zone wherein an eluent solution and an aqueous solution containing the acid and the other compound(s) are together passed through the contacting zones to chromatographically separate the acid from the other compound(s); and
(c) after step (b), sequentially processing the contacting zones through an elution zone so as to elute the desired product (the acid(s) or the other compound(s)) from the one or more contacting zones in a solution of the eluent. In advantageous processes, the eluent is water.
Another embodiment of the invention provides a polymer useful for the separation of an acid, comprising a bead-form, crosslinked polymer having repeating units selected from the group consisting of Formula I, Formula II and Formula III below: 
wherein:
m and n are each 0 or 1;
Y and Zxe2x88x92 are as defined above, with the proviso that when n is 0 in Formula I, Zxe2x88x92 is not Oxe2x88x92 (i.e. excluding N-oxide derivatives from Formula I);
R1 is an organic group having from 1 to about 6 carbon atoms, optionally containing one or more nitrogen or oxygen atoms; especially an alkylene group having from 1 to about 6 carbon atoms;
R2 is an organic group having 1 to about 12 carbon atoms and containing a negatively-charged oxygen, boron or sulfur atom, and a positively-charged nitrogen or phosphorous atom; especially a group selected from 
xe2x80x83wherein p is 0 or 1, R4, R5, R8 and R9 are alkyl groups, preferably having from 1 to about 6 carbon atoms, especially methyl, R6 is an alkylene group, preferably having from 1 to about 6 carbon atoms, R7 is an organic group such that the group xe2x80x94NH2xe2x80x94CH(R7) (CO2xe2x88x92) represents an amino acid, and n, Y and Zxe2x88x92 are as defined above; and
R3 is an alkyl group having 1 to about 6 carbon atoms, especially methyl.
The invention provides improved methods for separating acids from other compound(s), utilizing ampholytic base polymers, and also novel ampholytic base polymers useful for such separations. The preferred methods and polymers of the invention enable the conduct of improved procedures facilitating high levels of product recovery, and also not requiring the use of thermal swing conditions to achieve advantageous eluents having high product purity and concentration. Additional objects, as well as features and advantages of the invention, will be apparent from the following description.